The present invention relates to a process for producing an electric contact on a HgCdTe substrate of P conductivity, as well as an application of said process to the production of an N/P diode.
The invention is applicable to all devices using a contact on a P conductivity, HgCdTe substrate and particularly infrared detectors using N/P photodiodes.
The invention more particularly relates to the production of contacts on a HgCdTe substrate epitaxied on an insulating support. Thus, in this configuration, the surface of the substrate on which the contacts can be produced is reduced to the front face of the substrate in a P conductivity region. Thus, the contacts are of small size and must be produced in accordance with microelectronics integration processes. However, the invention obviously applies to the production of contacts on solid HgCdTe substrates.
In known manner, in order to produce an electric contact on a substrate, an insulating layer is previously deposited on the face of the substrate where the contact is to be formed. This insulating layer is etched through a mask, e.g. of photosensitive resin, using a chemical solution so as to produce an opening over the entire thickness of the insulating layer. A conductive material is then deposited in the opening made in the insulating layer in order to produce an electric contact on the substrate.
With this process, the slope of the etching effected in the insulating layer is dependent on the texture and roughness of the material constituting the insulating layer, as well as the composition of the chemical solution. Therefore the slope of the opening made in the insulating layer is generally abrupt. Moreover, the conductive material deposited in said opening can discontinuously cover the wall of the opening and that part of the substrate facing the opening, thus forming an open circuit with the HgCdTe substrate.
The contact on the substrate is even more delicate when the insulating layer is constituted by two-layer structures, e.g. of the SiO.sub.2 --Si.sub.3 N.sub.4, SiO.sub.2 --ZnS, or CdTe--ZnS types. These pairs of materials do not have the same etching rate and consequently one of the materials used for producing the insulating substrate is subject to more marked etching than the other, which leads to an opening having a discontinuous slope.
The object of the invention is to obviate these disadvantages by carrying out etching by ion bombardment of the insulating layer. This ion bombardment makes it possible to etch the materials currently used for producing an insulating layer and also to control the slope of the opening made in said insulating layer.
However, when ion bombardment reaches a P conductivity, HgCdTe substrate, the conductivity of part of the substrate affected by said bombardment is subject to a reduction of its P conductivity and its conductivity can become N conductivity. The conductivity reversal of part of a P conductivity, HgCdTe substrate by ion bombardment is described in U.S. Pat. No. 4,411,732. This reversal leads to a production of a N/P junction, which is particularly sensitive to infrared radiation. Unfortunately, an electric contact produced on that part of the substrate affected by the ion bombardment is not of an ohmic nature and generates noise.
Thus, a further object of the invention is to bring the conductivity of that part of the substrate affected by the ion bombardment to at least its initial value and to produce an electric contact on said P part of the "cured" substrate, said contact having an ohmic nature and which generates less noise.
The invention also aims at applying the process for producing a contact on a P conductivity, HgCdTe substrate to the production of an N/P diode.